According to asus, the b350m-a motherboard supports amd a10-9700, but the b450m-a does not, Why?
That is an AM4 processor so yes it will work on any AM4 motherboard
Some vendors removed support mainly as the ROM address space is tight, some boards are equipped with more ROM space
According to Asus Support as you mentioned, the A10-9700 is not listed as being supported.
The A10-9700 may fit perfectly in the Motherboard's socket but since there is no BIOS update to support that CPU it may not even boot up.
While the B350M-A as you mentioned that CPU is supported with a BIOS:
I suspect if the CPU iis installed it will work,
most machines have 16MB or larger ROMS so there is lots of room for device ID tables
I am not very technical concerning BIOS except from what I have learned on my own or through the various Forums. So what happens if none of the motherboard's BIOS versions indicates in Device ID's tables his CPU/APU because the Manufacturer left it out on purpose because they felt it may not be fully compatible with the hardware of the motherboard.
This is my logic concerning the answer I gave.
Explain why a motherboard won't boot up with a new version of Ryzen without first updating the BIOS that was created to make it compatible? In the past, motherboards were made before a new version of CPU came out but later on in its support site it shows the new version CPU will be compatible once you update BIOS created specifically for the new CPU. So logically it seems like the Motherboard must have the proper BIOS install to make the CPU to be recognized and compatible.
In the OP's Motherboard Compatibility list it doesn't mentioned the A10 as being compatible. So it is a good bet if the OP installs the APU it won't boot up or be recognized by the current BIOS version installed. Same reason you can't upgrade the CPU in laptops unless the manufacturer has a BIOS for the new CPU/APU even if its is the same type of socket.
IDK, maybe you have installed a CPU/APU in the past on a motherboard that is showing not listed as being compatible and it worked. But from my experience if the CPU/APU is not listed it generally indicates that it wouldn't work or work correctly. But I could be wrong. just my limited knowledge of BIOSes.
Besides doesn't the BIOS needs to be able to configure the new technical features of the new CPU/APU? If the new CPU/APU is not listed in BIOS and is not configured to use the new CPU/APUs features or technology I bet it won't work correctly if at all.
In my opinion, the best way is to open a Ticket with the manufacturer's Motherboard Support site to be certain any new upgraded CPU/APU will work especially if it is not listed.
AMD stated socket AM4 is to be well supported. Recall the flap over some MSI mistake.
Thank you for the information and advice, I think I will play safe and opt for the Asus B350m-a board.